540 
DE. CAEPEXTEE’S EESEAECHES OX THE FOEA^nXIFEEA. 
constitute the only interruption to the general uniformity of the surface, the septal 
bands hot being clearly distinguishable ; and this disposition is commonly found to 
prevail on the surface of the inner whorls, when it is exposed by the removal of the 
outer (n', ii). In the newer portion of the last formed whorl, on the other hand, we 
may observe that instead of each punctation having a separate dimple of its own, the 
corresponding punctations of the two rows lie in a succession of furrows that pass 
transversely between the septa {gg\ gg'). In the most recently foimed portions of 
specimens that have not attained their full growth, we find these furi'ows to be deeper 
towards the posterior than towards the anterior margin of each intei'septal space ; and 
in the deepest portion of each of these furrows, which obviously con’espond with the 
“ fossettes ” of P. crispa (although much less pronounced), a minute punctation may be 
brought into view by careful examination, — a con’esponding row of punctations being 
also traceable on the other side of the septum*. These varieties of superficial aspect 
may present themselves on difierent parts of one and the same specimen ; and it will 
appear from the explanations which I shall presently have to furnish, that they are 
occasioned by differences in the degree in which the proper external wall of the 
chambers is thickened by an exogenous deposit upon its surface, continuous with that 
of which the central nucleus is composed 185). 
181. Internal Structure . — By the fortunate contingency aheady adverted to. I have 
been enabled to study the internal structure of this remarkable type, not merely by 
examining thin sections of the shell taken in various dhections, and by comparing the 
appearances they present with those obtained by laying open its interior by fracture ; 
but also by submitting to microscopic examination siliceous “ casts ” of its carfties and 
channels, which appear to represent with the utmost fidelity the forms and comiexions 
of the various parts of the sarcode-body which occupied those carfties and channels in 
the living state of this organism. Between the results of these two modes of study I 
have found the conformity to be so exact, that the account of the structure of the 
animal which I should have given from examination of the shell alone, has not needed 
to be modified in the slightest particular by the information more recently furnished by 
these “ casts ; ” and, in point of fact, the ideal representation in Plate XVIII. fig. 1. which 
was entirely based on the former source of information, has not needed the least altera- 
tion to bring it into accordance with the exact delineations of the latter class of objects 
which are given in figs. 12, lof. 
182. In the general shape and proportions of its segments, this type of Polgstomella 
* In order to distinguish the orifices of these punctations, it is advantageous to remove from the surface 
of the shell that opacity which it derives from abrasion, and to get rid of the fine particles of calcareous 
matter which offceu choke up and obscure its pores. This is roadil}' eifected by immersing it for a short 
time in water so slightly acidulated with nitric or hydrochloric acid as only to exert a very feeble degree of 
solvent power. 
t I think it well to state this circumstance, as it may increase the confidence accorded to my descriptions 
of other types, of which the shells alone have been submitted to examination. 
